Well (1) you can be sure Musk is paying lawyers $$$ to make sure he doesn't deeply fuck up on any securities rules given his contentious relationship with the SEC [though fairly that might be de minimus to him] (2) there is an opportunity cost to putting money so much money in twitter just for laughs [he could be doing other things] and (3) there is risk that the twitter stock could fall out from under him. (3) is really only a cost relative to having the money in a diversified portfolio - but it is a real cost nonetheless.

He has put like $4 billion into Twitter stock, which is volatile.